This invention relates to wavelength add-drop multiplexing schemes and, more particularly, to the use of four-wave-mixing to implement such schemes.
The nonlinear optical response of semiconductor microcavities has attracted considerable attention recently. For instance, the time resolved measurement technique utilizing four-wave mixing (FWM) has provided the clear observation and systematic study of the normal mode oscillations in the THz regime [1]. (Note that in the above and following description, a reference""s identification [e.g., 1] refers to that reference""s location in the Appendix) In addition, the high finesse of microcavities allows enhancement of optical intensity inside cavities [2], which leads to higher nonlinear sensitivity and subsequent interesting phenomena: enhanced light diffraction, enhanced bleaching of absorption, THz oscillation of emission, etc. [3-5].
Non-degenerate four-wave mixing (ND-FWM) in semiconductor materials is an important optical parametric process that has also attracted increased current interest for applications. It provides the functionality of wavelength conversion required in the future fiber optic telecommunication technologies: for example, the all-optical wavelength conversion in the wavelength division multiplexing systems [6] and the mid-span spectral inversion in the long haul optical fiber cables [7]. Representative devices to realize such highly non-degenerate FWM signal generation in an efficient manner are optical waveguide devices, especially traveling wave semiconductor laser amplifiers [8-12, 18].
From such a viewpoint, it is interesting to study the ND-FWM phenomena in a microcavity and to investigate its potential as a wavelength conversion device. The advantages offered by the vertical cavity configuration, for instance, no cleaved facets, no anti-reflection coating and lower insertion loss, are attractive if the ND-FWM signal generation scheme works efficiently. Indeed, a degenerate FWM experiment with a GaAs microcavity has been reported and diffraction efficiency hd of 0.5% was demonstrated for 150 pJ pump pulses [3]. A ND-FWM experiment with a vertical cavity surface emitting laser was also performed [13]. The frequency shift in the latter was limited to 10 GHz, because only the resonance mode in the direction of surface normal was utilized. In those two cases, the degree of non-degeneracy was limited by the cavity resonance widths and, therefore, the non-degeneracy and enhancement of optical field are in a trade-off relationship.
Before such ND-FWM devices can be of practical use in optical communication systems they have to exhibit enhanced frequency shift and improved efficiency.
In accordance with the present invention, a new non-degenerate four-wave mixing (ND-FWM) method and apparatus are disclosed in which both pump and probe beams are incident at predetermined oblique angles and results in efficient highly ND-FWM signal generated from a microcavity device that includes a semiconductor quantum well. The phase mismatch originating from the unique relationship between a beam incidence angle and its microcavity resonance frequency is minimized when both of pump and probe beams are obliquely incident.
According to one feature of the invention, the relationship between the wavelengths of the pump xcfx89P and probe signals xcfx89PR and the xcex8P(i) and xcex8PR(i) oblique angles, respectively, is given by       ω    res    =                    ω        0                    cos        ⁢                  xe2x80x83                ⁢                  θ                      (            l            )                                =                  ω        0            ⁢                        1          +                                    "LeftBracketingBar"                                                k                  ∥                                                  k                  ⊥                                            "RightBracketingBar"                        2                              
where xcfx890 is the resonance frequency for the normal incidence, kxe2x8axa5 and k∥ are internal wave vector components normal and parallel to the microcavity surface, respectively, and xcex8"igr" is the internal incident angle corresponding to these wavevectors.
In other embodiments, the disclosed FWM techniques are used to implement a wavelength multiplexer and a demultiplexer. In yet other embodiments, the FWM techniques are used with a switched or pulsed pump signal to implement a packet wavelength multiplexer and a packet demultiplexer.